Diphenhydramine (the active ingredient in Benadryl) is generally considered acceptable to use during pregnancy at standard doses, but it comes with some caveats depending on when and how much you take. The FDA classifies it as Pregnancy Category B, meaning animal studies at up to five times the human dose showed no harm to the fetus, though well-controlled studies in pregnant women are limited.
That said, “not proven harmful” isn’t the same as “completely risk-free.” Here’s what the research actually shows about timing, dosage, and alternatives worth considering.
What Happens When You Take It
Diphenhydramine crosses the placenta rapidly. In animal studies using instrumented pregnant sheep, peak drug levels appeared in fetal blood within five minutes of the mother receiving the drug. The fetus was exposed to roughly 85% of the maternal blood concentration, which is significant. Because the drug is fat-soluble, researchers believe human placental transfer is similar. This doesn’t automatically mean harm, but it does mean your baby is exposed to nearly the same amount of the drug you are.
First Trimester: Birth Defect Concerns
The biggest worry for most people is whether taking diphenhydramine early in pregnancy could cause birth defects. A large study from the National Birth Defects Prevention Study compared first-trimester antihistamine use in over 13,000 infants with birth defects against nearly 7,000 healthy controls. For diphenhydramine, the results were largely reassuring. Previously suspected links to cleft palate and cleft lip did not hold up after adjusting for other factors. The cleft palate association, for example, had a confidence interval that crossed the threshold for statistical significance, meaning the data couldn’t confirm a real connection.
One finding did stand out: an exploratory analysis found a possible link between diphenhydramine and a specific heart defect called transposition of the great arteries, with exposed infants about 2.3 times more likely to have it. But this was an exploratory finding, not a pre-planned hypothesis, which makes it less reliable. Overall, the study’s authors concluded their findings “do not provide meaningful support for previously-posited associations between antihistamines and major congenital anomalies.”
Third Trimester and Late Pregnancy Risks
Later in pregnancy, diphenhydramine carries different concerns. There are reports of increased uterine contractions and, in very rare cases, fetal death associated with third-trimester use. These reports typically involve doses higher than recommended or prolonged daily use rather than occasional standard doses.
Babies whose mothers took diphenhydramine daily throughout pregnancy have occasionally shown withdrawal-like symptoms after birth, including tremors and diarrhea. These are temporary, and not every exposed newborn experiences them. The risk appears tied to consistent daily use rather than occasional doses for a bad allergy day or a sleepless night.
Using It for Sleep
Many pregnant women reach for diphenhydramine not for allergies but for insomnia, since it’s the same ingredient in over-the-counter sleep aids like ZzzQuil and Unisom SleepGels. A randomized controlled trial found that pregnant women taking diphenhydramine had meaningful improvements in sleep duration and sleep efficiency compared to placebo at both two and six weeks.
If you’re using it for sleep, keep in mind that nightly use increases your overall exposure, which is where the withdrawal risks for newborns come into play. Cognitive behavioral therapy for insomnia (CBT-I) is the preferred first-line approach for pregnancy-related sleep problems, and many pregnant women prefer it over medications. If you’re losing sleep regularly, that’s worth discussing with your provider rather than relying on diphenhydramine nightly for weeks or months.
One important safety note: combining diphenhydramine with certain other sleep medications has been associated with fetal death. Never layer multiple sedating drugs during pregnancy.
Alternatives Worth Considering
If you’re taking diphenhydramine for allergies, newer antihistamines may be a better fit during pregnancy. The American College of Obstetricians and Gynecologists lists several antihistamines as safe in pregnancy, including cetirizine (Zyrtec), loratadine (Claritin), chlorpheniramine, and hydroxyzine. These are typically recommended as first-line options for allergy symptoms in pregnancy.
Cetirizine and loratadine have the added advantage of not causing drowsiness, which matters when you’re already dealing with pregnancy fatigue. They’re also longer-acting, so you take them once a day instead of every four to six hours. If your main reason for reaching for diphenhydramine is a stuffy nose or seasonal allergies, switching to one of these is a straightforward choice.
Keeping the Dose Standard
The safety data on diphenhydramine in pregnancy applies to standard recommended doses. Most of the concerning case reports involve higher-than-recommended amounts or extended daily use over weeks to months. Occasional use at the dose listed on the package for a night of terrible allergies or an acute reaction is the scenario where the risk-benefit balance is most favorable. Taking it every night for your entire pregnancy is a different calculation, with a higher chance of newborn withdrawal symptoms and greater total fetal exposure.